The invention concerns piezoceramic multilayer actuators and a process for their manufacture.
Piezoceramic multilayer actuators are manufactured as monoliths, that is to say the active material onto which internal electrodes are deposited by a silk screen process prior to sintering, is disposed as a so-called green film in successive layers as a stack that is compressed into a green body. The compression of the green body is usually carried out by lamination in laminating moulds under the action of pressure and temperature.
A piezoceramic multilayer actuator 1 manufactured in this manner is shown schematically and much enlarged in FIG. 1. The actuator consists of stacked thin layers 2 of piezoelectrically active material, for example lead zirconate titanate (PZT), with conductive internal electrodes 3 disposed between said layers and which are led out alternately at the surface of the actuator. External electrodes 4, 5 connect the internal electrodes 3. As a result, the internal electrodes 3 are electrically connected in parallel and combined into two groups. The two external electrodes 4, 5 are the connecting poles of the actuator 1. They are connected via the connections 6 to a voltage source, not shown here. If an electrical voltage is applied via the connections 6 to the external electrodes 4, 5, this electrical voltage is transmitted in parallel to all internal electrodes 3 and produces an electric field in all layers 2 of the active material, which is consequently mechanically deformed. The sum of all of these mechanical deformations is available at the end faces of the head region 7 and the foot region 8 of the multilayer actuator 1 as a useable expansion and/or force 9.
FIG. 2 shows a section through the external electrode 4 and the surface of the piezoceramic multilayer actuator 1 according to the prior art. The structure of the external electrode can be seen in this Figure. In the region where the internal electrodes 3 are led out at the surface 10 of the multilayer actuator 1, a base metallisation 11 for connecting the internal electrodes 3 of identical polarity is deposited on the thin layers 2 of the piezoelectrically active material compressed into a stack, for example by means of an electroplating process or silk-screening of metal paste. This base metallisation 11 is reinforced by a further layer 12 of a metallic material, for example by a structured sheet or a wire mesh. The connection of the reinforcing layer 12 with the base metallisation 11 is achieved by means of an interconnecting layer 13, usually a solder layer. An electrical connecting wire 6 is soldered to the reinforcing layer 12.
External electrodes on the surface 10 of an actuator 1, which are constructed as described, have a serious drawback. During operation, large tensile stresses act upon the inactive region, the insulating region 14 that lies under the base metallisation 11. Since this insulating region 14 forms a homogeneous unit together with the base metallisation 11 and the interconnecting layer 13, this fails when the tensile strength of the weakest element is exceeded and cracks develop. The described crack progression occurs after about 106 load cycles. Because of the stresses occurring, the cracks 15 usually run from the brittle and low tensile strength base metallisation 11 into the insulating region 14 and are trapped therein by regions with high tensile stresses, preferably at the electrode points 16 of the electrodes 3 not in contact with the base metallisation 11, or they start in the regions of maximum tensile stress at the electrode points 16 and run in the direction of the base metallisation 11. The propagation of a crack 17 along the internal electrode 3 contacting the base metallisation 11 is classified as non-critical since such a crack progression does not impair the operation of the actuator. On the other hand, cracks 15 which run unchecked through the insulating region 14 are very critical since they reduce the insulation distance and seriously increase the probability of actuator failure due to flashovers.
Solutions to problems are described, for example, in the patent applications DE 198 60 001 A1, DE 394 06 19 A1, DE 196 05 214 A1. Here it is proposed that the region between an electrode not contacting the base metallisation and the base metallisation, be provided with a filler material of low tensile strength, or a hollow space. The main disadvantages of this procedure to be considered here are that the filler material must be introduced by means of an additional, complex process step, that the filler material inevitably has a negative effect on the properties of the actuator and, in the case of the hollow spaces introduced, these have to be re-closed in a further process step prior to the deposition of the base metallisation.
Another solution to the problem is proposed in DE 199 28 178 A1. Here the monolithic construction is split into small partial regions and reconstructed with alternating, inactive, electrode-free regions. By doing this, the maximum possible tensile stress within an active region is intended to remain below the value necessary for crack formation. From a manufacturing standpoint, the process is difficult and does not result in the necessary reduction in stresses in the isolating region, so that there is always a potential risk of cracking.
The object of the invention is to design the multilayer actuators so that the causes leading to crack formation in the multilayer actuators are avoided as far as possible.
The object is achieved in that a pattern is produced on the surface of the inactive region, the insulating region, by erosions interrupting the surface. The base metallisation is deposited exclusively on the surface left by the pattern. As a result, the external electrode is not connected to the entire surface of the multilayer actuator thus reducing the stiffness of the composite structure, comprising the surface of the isolating region, the base metallisation, the interconnecting layer and the reinforcing layer. The effect of the patterning is that the mechanical repercussions of the external electrode on the actuator is reduced. The tensile stresses occurring in the isolating region can no longer add up to a critical value exceeding the strength of the ceramic material and causing cracks.
An optimum effect is obtained if the depth of the erosions at the surface of the actuator producing the pattern corresponds to the thickness of the insulating region. The material of the insulating region is continuously interrupted so that stresses occurring cannot be transferred and as a consequence cannot add up to a critical value. The thickness of the isolating region depends, among other things, on the size of the actuator. Where the thickness of the insulating region is fully utilised, the depth of the structure, the depth of the erosions, can be up to about 0.5 mm.
The pattern can be applied to an actuator both in the green state and in the sintered state. Processing in the green state is the most advantageous one because, due to the softer ceramic material, it causes less tool wear and may also be carried out with processes other than the usual grinding process. A further advantage is that the sinter skin produced during sintering covers and isolates the structure deposited in the green body. As a result, the structure is reliably protected against electrical flashovers and moisture.
The interruption of the surface of the insulating region by erosions, in particular in the green state, can easily be effected by any machine cutting process. When an actuator is in the sintered state, in addition to grinding, machining with a suitable laser also presents itself. The surface processed by grinding after sintering must of course be subsequently insulated, for example by a polymer material.
The simplest erosion of the surface is the incorporation of linear structures, in particular grooves in the form of channels or notches, because they are also suitable for a specific structuring of relatively large areas. In this case the grooves can run in parallel.
The width of a groove, the spacing between two grooves and the angle of these grooves with respect to the longitudinal axis of the actuator must be matched to one another, so that no more than a predetermined number of internal electrodes occurs between two grooves at the surface of the actuator. This maximum number depends on the specific expansion of the actuator in operation and the strength of the ceramic material. The grooves can be disposed at a spacing of 0.2 mm to 10 mm and adjusted to the size of the actuator. A spacing of approximately five layers of internal electrodes or a spacing of approximately 0.8 mm to 1.2 mm between two grooves has proved particularly suitable. If the number of internal electrodes is made too high, the tensile stresses add up from electrode to electrode and exceed the critical value, this leading to the formation of cracks.
The grooves can be disposed at an angle of 0 degrees to 80 degrees to the longitudinal axis of the actuator. At 0 degrees the grooves run parallel to the longitudinal axis of the actuator. A range of 50 degrees to 30 degrees is advantageous. If 45xc2x0 is chosen as the angle of intersection, for example, then a favourable parameter is a spacing between two grooves of approximately 0.7 mm and a groove width of approximately 1 mm/2.